Manufacture of silver chloride electrodes



Oct. 31, 1961 H. E. HARING 3,006,821

MANUFACTURE oF SILVER cHLoRIDE: ELECTRGDES Filed March 29, 1945 SILVERCHLOR/DE S/L YER C/ILGR/DE SIL VE R CHLOE/0E VA W;

/Nl/EN TOR H. E. HA R/NG United States Patent O M 3,006,821 MANUFACTUREOF SILVER CHLORIDE ELECTRODES Horace E. Haring, Summit, NJ., assignor toBell Telephone Laboratories, Incorporated, New York, N.Y., a

corporation of New York Filed Mar. 29, 1945, Ser. No. 585,417 6 Claims.(Cl. 204-38) This invention relates to methods of forming electrodes forelectric cells. More particularly, this invention relates to methods offorming silver chloride electrodes in which the silver chloride iscarried on a metallic base, particularly a base of metallic silver.

Silver chloride electrodes have been proposed as cathodes in electriccells using various electrolytes and -anodic materials. The presentinvention provides convenient methods of manufacturing such electrodesand of manufacturing electrodes of novel structure which are capable ofcarrying much higher current densities than the electrodes hithertoproposed and which become fully active as to their current carryingcapacity and their contribution to cell voltage much more quickly thanhas been heretofore possible.

Silver chloride is a substantially non-conductive material. 'Thereforeelectrodes embodying this material must have a current collectingframework of conductive material. Although it is commonly supposed thatthe reduction of silver chloride to silver takes place directly in the`operation of silver chloride cells, it can be demonstrated that thereduction of the electrode takes place through the continuousdissolution of the silver chloride in the electrolyte immediatelyadjacent the conductive base and the simultaneous deposition of thesilver ions as metallic silver upon the conductive base, the chlorideions remaining in solution.

Therefore, the reduction of la silver chloride electrode must be treatedas a plating process. Since the current generating capacity of theelectrode in a cell is dependent upon the rate at which the metallicsilver is plated, it is essential 4that large -areas of the conductiveframework be exposed yto the electrolyte and that large areas of silverchloride exist in contact with the electrolyte in close proximity to theconductive framework if a virtually instantaneous peak cell voltage andhigh current carrying capacity are required.

According to the preferred procedure of the present invention, a silverchloride electrode is formed by depositing a layer of silver chlorideupon a silver base, such as a screen or sheet having a silver surface byimmersing the silver base as an anode in a solution containing chlorideions. Although preferably the silver base is solid silver, ya base ofsome other metal, such as copper, having a relatively heavy plate ofsilver may be used. 'Ihe quantity of silver chloride formed, andtherefore the capacity of the electrode, is determined by the number ofampere minutes of formation. After the desired number of ampere minuteshave passed through the silver base the anodizing current isdiscontinued.

Ihe electrode made up of a silver base carrying silver chloride is thentreated to produce exposed areas of metallic silver electricallyconnected by silver filaments to the silver hase. These exposed areas ofmetallic silver are joined by lightly silvering the entire surface ofthe electrode and this conductive system together with the silver baseserves as the current collecting framework referred to above.

In `order to compact the silver chloride coating `and give it mechanicalstrength, the electrode is subjected to a high mechanical pressure, asin a hydraulic press. IThis pressing operation may be carried out at anytime after the formation of the silver chloride coating.

j 3,006,821 Patented Oct. 31, 1961 ICC To produce an electrode capableof generating an extremely high current density, a porous silver lbasesuch as a ne mesh of ne silver wire is usually used. This base,preferably -a silver screen, may be suspended as an anode in -a suitableelectrolyte as described above until the desired amount of silverchloride has been deposited. The most satisfactory results uponanodizing are obtained if the silver base, and particularly silverscreen, is annealed prior 'to anodizing. Annealing at a temperature ofabout 600 C. for about thirty minutes has been found suitable. In theabsence of annealing, non-uniformity of the met-a1 due to mechanicalstress in manufacture causes non-uniform anodizing. The anodizingprocedure and -the changes taking place in the various stages of ltheprocess of preparing the electrode Iare illustrated in the accompanyingdrawing in which:

IFIG. 1 is a front elevation, partly in section, of an vapparatus whichmay be used for anodizing;

FIG. 2 is a diagrammatic representation, in section, of a portion of asilver screen prior to anodizing;

FIG. 3 is a diagrammatic representation, in section, of the same silverscreen after anodizing;

FIG. 4 is a diagrammatic representation, in section, of the anodizedsilver screen of FIG. 3 after silver filaments have been caused to growout from the base silver by cathodic reduction;

FIG. 5 is a diagrammatic representation, in section, of the anodized andthen cathodically treated screen of FIG. 4 after silvering by immersionin a reducing solution;

FIG. 6 is a diagrammatic representation, in section, of the body in FIG.5 after being subjected to mechanical pressure;

FIG. 7 is a front elevation of -a finished electrode prepared accordingto the process of lthe present invention.

In FIG. l the silver screen 1 or other silver body is suspended by meansof the rack 2 and clamps 3 in the electrolyte 4 contained in theinsulated tank 5. The mesh is connected to a suitable current source bythe conductive means 6. Suitable electrodes of opposite polarity (notshown) are suspended in the electrolyte in such position as to provide`a uniform current distribution over the surface of the silver body.

Any suitable aqueous electrolyte containing chloride ions may beemployed in the anodizing operation. An aqueous solution of sodiumchloride or hydrochloric acid has been found satisfactory. Theconcentration of the solution should be sucient to give the desiredconductivity. The upper limit of the concentration is set only by thevalue at which the solubility of the silver chloride formed becomesundesirably high at the temperature of Ithe solution so that the rate ofdeposit of silver chloride becomes too slow and an undesirable -amountpasses into the solution. Ordinarily an aqueous solution in which thechloride ions constitute in the vicinity of 2 percent by weight of thesolution will be found satisfactory.

In order -to increase the rate at which -the formation of silverchloride takes place it has been found desirable to heat the electrolyteabove room temperatures. Temperatures between about 60 C. and 80 C., andpreferably of about C. are the most suitable. This use of elevatedtemperatures also considerably improves the physical properties of thedeposit.

Another expedient which may be employed to decrease still further thetime required for anodizing is to add to the electrolyte a lowconcentration of anions which will form with silver a compound moresoluble than silver chloride. Fluoride ions or preferably nitrate ionsmay be employed for this purpose. Nitrate ions may be added in the formof nitric acid at the rate of 5 cubic centimeters of concentrated nitricacid per square foot of screen.

The anodizfing potential and current density are not critical and themost desirable values can readily be determined by those skilled'in theart. A potential of about. 1.8 volts has been found satisfactory Vforall `pur- Y poses.. The anodizingis continued until Slightly more thanthe .desired number of ampere Vminutes to be generatedfby the' cell'havepassed through the screen.

- `Inlorder to provide a convenient means for electrical connection toVthe finished electrode, it is desirable that a portion of the silverbaseV remain uncoated with silver chloride. Thisv is accomplished mostsimply by coating such portions. with a non-conductive lacquer. 4Thisexpedient 'is desirable even when the portion which is to remainunanodized extends out of theelectrolyte, since in the absence of aVlacquer coating at the waterline the electrolyte tends to creep up Vthesilver baseand evaporate, thus forming aline of high electrolyteconcentration at which the anodizing proceeds at 'a fastenrate thanbelow the surface of the electrolyte bath; .This Vrapid anodization atthe water line causes the silver base to be converted too rapidly thusresulting in a line -of mechanical weakness. .A coating of lacquerextending well yabove and below theewater line avoids this diiculty.After the preparation of the electrode has vbeen completed, the lacquercoating may ibe removed by mechanical .means to the extent necessary to.permit good electrical contact. FIGS. 2 to 6 showV inY a -purelydiagrammaticrmanner the changes taking place during the various stagesof the preparation of the electrode. The `outer sections ofthe wires 7-of the initial silver screen shown in FIG. y?, are, during the anodiingoperation referred to above, converted to a silver chloride layer v8, as`shown in FIG. 3. `After being anodized the scree'n is suspended for ashort period as a cathode in any suitable electrolyte which will not-have a harmful effect on the silver chloride coating.

This cathodizing operation Ymay be carried out simply by reversing thepolarity of the.v electrodes in the apparatus shown in FIG. 1.Preferably, Y however, the screen is removed from this apparatus andsuspended as Y a cathode Vin an aqueous solution of sodium chloride 'forthe required period of time. A solution containing about 5` percent ofsodium chloride has been found very suitable. Obvious, otherelectrolytes may be employed.

This cathodizing operation usually Yis carried out at or above thecurrent density at which vthey cathode is intended .to be dischargedinthe cell .in which vit is to be employed. Thisoperation is made verybrief, e.g. 4 to 5 seconds, so that only -a minimum of the capacity ofthe electrode is destroyed by reduction of the silver chloride tometallic silver. e

During -this'operatiom iilamentary bridges 9 of porous metallic silverconnecting the wires 7 `of the mesh with ythe outer surface of thesilver `chloride layer are formed by theV reduction of the walls ofpores through the silver chloride coating, as shown Yin FIG. 4. At thesurface otY the silver' chloride these lamentary `bridges tend to spreadout toa certain extent, creating the appearance of snowflakes.V Y p YThe screen is then Yimmersed in a suitable reducing agent'adapted'toreduce chemically the entirevouter surface of the silver chloride to aconductive layer 10 of Vporous silver Vasshownin FIG. 5. Y This reducedsurface 10 connected by the bridges 9 to the inner silver'wires 7Yconstitutes a very effective current collecting framework which willenable the immediate generation of extremely high currentrdensities incells employing this electrode.

One of the most effective reducting agents for the formation of thisconductive layer on the silver chloride is an aqueous solution ofhydroxylamine. One reason for the particular effectiveness of thissubstance is the fact that a quantity of ygas is continuously generatedat the surface of the silver chloride as a product ofthe re ducingaction. The evolution of this gas Yserves to induce local agitation ofthe reducing solution making unnecessary the supplementary mechanicalagitation required for best results with other reducing agents.

Other suitable reducing agents are aqueous solutions of any of thecommon photographic developers, such as p-aminophenol, o-aminophenol,amidol (ZA-diaminophenolV hydrochloride), metol `(p-rnethylamirlophenolsulfate), catechol or hydroquinoue. The concentrations which are commonfor photographic developing are suitable and the pH of the solutionsshould be adjusted as in photographic developing solutions. Immersionfor one to three minutes is ordinarily satisfactory. The exact timedepends upon the dilution, temperature and age of the developingsolution. Y

A particularly effective reducing solution of the photographic developertype contains in each liter of aqueous solution, approximately 1.5 gramsof hydroquinone, 0.5 gram of elon (p-methylarninophenol sulfate), 6grams of anhydrous sodium sulte, and 9 grams of anhydrous sodiumcarbonate. 'Y l 'Y In order to consolidate and impart mechanicalstrength to the silver surfaced silver chloride coating, the mesh issubjected to a high'mechanical pressure, such as three to four tons persquare inch, to form a compact electrode, e.'g. two-thirds its formerthickness. The iinished electrode has the appearance of a solid silversheet ecked withV silver snowflakes caused yby the arrangement of thesilver bridges as illustrated in 7.V .In this figure the edge 11consists of uncoated silver screen and -rnay be used Vfor makingVelectrical contact with the electrode. The remainderf12 of the electrodeis coated with silver chloride and reduced silver as described above.The crss-sectional appearance of Vthe nished sheet isrepresenteddiagrammatically in FIG. 6. Electrodes having this structureand cells containing them are more particularly described Vand claimedin the application of H. E. Haring, Serial No. 585,418, filed on thesame day as the present application.

AsV indicated above the pressing operation may be performed at any timeafter the anodizing operation. Thus the electrode may be subjected tomechanical pressure immediately after anodizing and before thesubsequent reducing operations, or it can be pressed after cathodicreduction and before the chemical'surface reduction or itcan be pressedafter all three operations have been completed. When the initial silverchloride coating is applied by a procedure, other than yanodizing,fwhichV yields a compact mechanically durable layer of silver chloride,it is obvious that no further pressing 'operation 'is required. Y

VThe manufacture of electrodes has been described above'as applied tosilver screen. The process of the present invention is applicable toother silver bases such fa'svsilver sheets, either perforate orimperforate, and grids Yof silver or silver'plated metal. A desirable-base for `s'o'me purposes consists ofV silver sheets pierced at regularintervals with small holes, the -metal displacedy by the piercingoperation projecting `inthe form fof'sharp protberances from the surfaceof the sheet so as to resemble a grater.. Y p, The Jprocedures ofelectrolytic and chemical reduction described abovemay be carried outwith silver bases coated with silver Achloride by methods other thananodizfing. Any coating` procedure which produces a coating Awhich isvsufficiently porous to permit electrolytic reduc- .Ytion of thewalls-of the pores `to form the required la- .mentary'bridges of silver,as vdescribed above, will be suitable.- Thus a suitable silver chloridecoating may be dformed on silver screen'by pressing, Ypreferably hotpressingya layer-of finely VdividedI silver'chloride onto the surface.An `adequate coating can also be produced by rpasting* Apowdered 'silverchloriderinto a silver grid with a suitable'binder such as gum arabic.

f The invention has' been described in Vterms of its Vspe- -cicembodiments, which are to'berconside'red illustrative only and not to be.a"flimitation"f.upon'the scope of the invention, which is to be limitedonly by the scope of the appended claims.

What is claimed is:

1. The process of preparing an electrode for an electric cell whichcomprises anodizing the surface of a silver body by passing an electriccurrent, in a direction such that the body acts as an anode, between acathode and said silver body through an aqueous electrolyte containingchloride ions while the silver body is immersed therein until a layer ofsilver chloride has been deposited on the surface of said body, passingan electric current, in a direction such that said body acts as acathode, between said anodized silver body and an anode -through anaqueous electrolyte in which the silver body is immersed until a portionof said silver chloride has been reduced to form a plurality ofconductive iilarnentary bridges of metallic silver extending between thesurface of the silver body and the external surface of the silverchloride layer and immersing said body in a solution of a reducing agentcapable of reducing the silver chloride to metallic silver upon contactuntil a thin layer of porous metallic silver has been formed von thesurface of the silver chloride.

2. The process as described in claim 1 including the step of compactingthe deposited silver chloride layer against said silver body bymechanical pressure at some stage of the process after the initialformation of the silver chloride layer.

3. The process as described in claim 1 wherein the aqueous electrolyteused for :anodizing is maintained at a temperatnre between about 60degrees C. and about S0 degrees C. during the anodizing operation.

4. The process as described in claim 1 wherein the aqueous electrolyteused for anodizing contains nitrate ions.

5. A process for preparing an electrode for an electric cell whichcomprises anodizing the surface of an annealed silver screen by passingan electric current, in a direction such that the screen acts as ananode, between said screen and a cathode through an aqueous electrolytecontaining chloride ions while said screen is immersed therein until alayer of silver chloride has been deposited on the surface of saidscreen, said electrolyte being maintained at a temperature between aboutdegrees C. and about 80 degrees C. during said anodizing operation,passing an electric current, in a direction such that said screen actsas a cathode, between said anodized silver screen and a cathode throughan aqueous electrolyte in which said screen is immersed until a portionof said silver chloride has been reduced to form a plurality ofconductive iilamentary bridges of metallic silver extending between thesilver screen and the external surface of the silver chloride layer andimmersing said screen in a solution of a reducing agent capable ofreducing silver chloride to metallic silver upon contact until a thinlayer of porous metallic silver has been formed on the surface of saidsilver chloride.

6. The process described in claim 5 wherein the solution of reducingagent is an aqueous solution of hydroxyl amine.

References Cited in the tile of this patent UNITED STATES PATENTS 7,821Mathiot Dec. 10, 1850 26,978 Edwards lan. 31, 1860 303,237 SkrivanowAug. 5, 1884 405,196 Barrett et al. June 11, 1889 434,593 Fink Aug. 19,1890 512,055 Pearson Jan. 2, 1894 1,442,238 Smith Jan. 16, 19232,422,323 Waltman June 17, 1947 OTHER REFERENCES Trans. Electrochem.Soc., vol. 72 (1937), pages 63.

1. THE PROCESS OF PREPARING AN ELECTRODE FOR AN ELECTRIC CELL WHICHCOMPRISES ANODIZING THE SURFACE OF A SILVER BODY BY PASSING AN ELECTRICCURRENT, IN A DIRECTION SUCH THAT THE BODY ACTS AS AN ANODE, BETWEEN ACATHODE AND SAID SILVER BODY THROUGH AN AQUEOUS ELECTROLYTE CONTAININGCHLORIDE IONS WHILE THE SILVER BODY IS IMMERSED THEREIN UNTIL A LAYER OFSILVER CHLORIDE HAS BEEN DEPOSITED ON THE SURFACE OF SAID BODY, PASSINGAN ELECTRIC CURRENT, IN A DIRECTION SUCH THAT SAID BODY ACTS AS ACATHODE, BETWEEN SAID ANODIZED SILVER BODY AND AN ANODE THROUGH ANAQUEOUS ELECTROLYTE IN WHICH THE SILVER BODY IS IMMERSED UNTIL A PORTIONOF SAID SILVER CHLORIDE HAS BEEN REDUCED TO FORM A PLURALITY OFCONDUCTIVE FILAMENTARY BRIDGES OF METALLIC SILVER EXTENDING BETWEEN THESURFACE OF THE SILVER BODY AND THE EXTERNAL SURFACE OF THE SILVERCHLORIDE LAYER AND IMMERSING SAID BODY IN A SOLUTION OF A REDUCING AGENTCAPABLE OF REDUCING THE SILVER CHLORIDE TO METALLIC SILVER UPON CONTACTUNTIL A THIN LAYER OF POROUS METALLIC SILVER HAS BEEN FORMED ON THESURFACE OF THE SILVER CHLORIDE.